Hot answers tagged

22

There's always a ground. Whether it's what you intend it to be or not is another issue... A mag-mount antenna is grounded through capacitive coupling between that magnet and the metal it's stuck to. At VHF/UHF frequencies, this effect is adequate for good results which explains the popularity of these mounts. Some folks advocate adding a wire instead of ...


18

The bits per sample will affect the dynamic range of your receiver. There's a lot of math that I'm sure you can find, but here's the intuitive explanation: A digital signal can represent only discrete quantities, where an analog signal can represent infinitely many quantities between any two discrete quantities the digital signal might represent. The ...


16

When the tone is present, it is because the antennas are out of phase. One antenna's signal will be leading the other; which one is leading depends on which one is closer to the transmitter (provided that the antennas are less than 1/2 wavelength apart, so that the phase difference will always be less than 180°). When we switch from the lagging signal to ...


14

What do you mean by "circuit"? Do you mean there's a loop of conductor from one side of a battery to the other, possibly with some other conducting components along the way? How about this circuit? simulate this circuit – Schematic created using CircuitLab Is there a circuit here? There certainly isn't any way to follow a line from one side of the ...


12

We use decibels because they are the most intuitive representation when our primary concern is signal quality. Signal quality changes not in proportion to the power added (in watts), but rather to the percentage change. Decibels put "percentage change" on a linear scale which is easy and intuitive. I can't show you with a radio here, but I can show you with ...


11

Normally in amateur radio when specifying a frequency you specify the nominal carrier frequency. For SSB and other suppressed-carrier transmission modes, you specify the frequency to which the BFO needs to be tuned to re-insert the suppressed carrier. For this to work, you also need to specify which sideband you are transmitting on; lower, upper, or both. (...


11

It depends on what you mean by "stronger". Ignoring actual limitations of your hardware and just considering the theory of communications, if you have 25 W of transmit power, then you can spread that over as much bandwidth as you want, and you're always transmitting 25 W of power. The quantity which is 25 times higher in the 100 Hz case than the 2500 Hz ...


10

Satellites can and do use HF for communications. The first example would of course be Sputnik, which transmitted at 20 and 40 MHz. Amateurs use HF to communicate with satellites. According to Amsat, mode A: This mode requires a 2 meter SSB/CW transmitter and a 10 meter SSB/CW receiver... Mode K; This mode requires a 15 meter SSB/CW transmitter and a 10 ...


8

The ionosphere typically neither reflects nor absorbs waves with VHF or higher frequencies, but passes them through to space. There are no reflections back to the ground, so there is no useful propagation between stations on the ground. Tropospheric ducting is a different propagation mode which does carry VHF signals well, and is actually sought and used ...


8

Switching to the inverted signal is equivalent to multiplying by -1. While in an ideal mixer, the RF input can be multiplied by a sinusoidal LO, the switching mixer multiplies by a square LO. It still works as any mixer would. That is, for each frequency component in the first input $f_1$, and each frequency component in the second input $f_2$, it produces ...


7

A coaxial cable is a transmission line by itself, and therefore the velocity factor is known. It depends on the dielectric properties, geometry, and conductivity. A single wire is not a transmission line. You need a return path for the current, and that will most probably be a second wire somewhere, or a ground plane. So the geometry and dielectrics around ...


7

It's easier to see what's going on with a bit of rearrangement. Imagine this built of tubing: The feedpoint is still where it would be on a dipole without the balun. The feedpoint sees the dipole as usual (blue). In parallel with that is a twin-lead transmission line formed by the two parallel sections of the balun (red). This is a quarter-wave section ...


6

You raise an excellent question and your thought processes are indeed on the right track. First some background. An ideal, uninterrupted sinusoidal carrier has zero bandwidth. Real world factors such as phase noise, amplifier distortion, etc. produce a measurable bandwidth of the carrier. When the carrier is keyed on and off as it is with Morse code, this ...


6

Faraday cages block EMP in the same way they block all other time-varying electromagnetic fields. The only difference between blocking EMP versus blocking an ordinary radio transmission is the EMP is many orders of magnitude stronger. A Faraday cage works because metals consist of a "sea" of mobile electrons among the protons in the atomic nuclei. That is, ...


6

The short answer is that it can't. A shielded transmitter, connected to an ideal piece of coax, does not generate common mode currents. The inner and the outer of the cable look connected (and for Direct Current they are), but high frequency currents really cannot pass through the thick metal, they are confined to the inside or the outside. Real coax can ...


5

Twin-lead transmission lines don't radiate because the opposite fields from each conductor cancel, but when the spacing is far apart this does not happen. First, let's consider the magnetic field around an infinite, straight conductor with uniform current throughout. In this image, the conductor is just right of center, and the current is coming straight ...


5

SSB is, as you say, half of AM, without a carrier. That's the way to think about it if you are implementing it with traditional analog electronics. However, here's a simpler way to think about it: SSB is the baseband signal shifted up into RF. Human hearing works from about 20Hz to 20,000Hz. Human voice needs only up to about 4000Hz. So, take your voice ...


4

FM works by varying the frequency of the signal around the nominal carrier frequency. Because the frequency is varying, the signal is not a pure sine wave. Therefore, it necessarily has some energy in sidebands as well as the instantaneous frequency. The higher the audio frequency, the more the signal deviates from a sine wave, so the more energy ends up in ...


4

The number of bits in the converter will set the maximum dynamic range of the resulting data stream. This is approximately $\textrm{SNR} = 6\,\textrm{dB} \times \textrm{Bits} + 4.8\,\textrm{dB}$ (for a full scale input sine wave). However, as with everything in life, this is only the beginning of the story. The data sheets for the converters will typically ...


4

I'm afraid that it is not correct that there is no energy transfer back to the source after an initial period. Wherever you have a mismatch on a transmission line, there is a reflection (at least partial) back to the source. A percentage of power is "actually" reflected back. It can be separated from the transmitted power using a suitable device like a ...


4

There is no way to transmit information in a signal with zero bandwidth. Switching the carrier at the zero crossings would reduce bandwidth but not take it to zero. There's a mathematical explanation on DSP StackExchange: Does “keying on” a sine wave at a zero-crossing reduce its bandwidth? Summary: in the worst case of switching on/off at the peak ...


4

It is helpful to understand the basic functioning of a coaxial cable. But first there are two important phenomenons that must be understood in order to proceed. Skin Effect When direct current (time invariant current) passes through a conductor it tends to uniformly use the entire cross sectional area of the conductor. When alternating current (time ...


4

Current flow is electron flow and it is from negative to positive. This is what we call class A hogwash. Current notation is just a convention. Going by electron flow is not righter than going the other way around. Conventional current flow is mostly used in Electrical Engineering. As an EE, can confirm. In Radio there is a greater tendency to do ...


4

The E and H fields of all RF signals travel at exactly the same speed - the speed of light. The phase relationship between the two fields remains constant as they travel through various mediums. Lightning in itself is not an RF signal but it does emit RF (with E&H waves) as it propagates. [EDIT] After reading some of the additional dialog provided by ...


4

A transmission line is linear, which means we can consider the voltage at any point to be the superposition of two other voltages. Asking what they are "physically" may or may not be a useful question. I could for example define: $$ V(z) = Ve^{jkz} + V_{DS}(t) $$ where $V_{DS}(t)$ is a function corresponding to the normalized amplitude of Pink Floyd's ...


3

The two are separate figures, so one does not imply the other. A good match doesn't imply no common mode currents, and no common-mode current does not imply a good match. They are somewhat connected - a change to the common mode currents will probably change the impedance match. But the other way doesn't have to apply - imagine soldering a small capacitor ...


3

For HF usage with 200 Msps converters and above, an 8 bit ADC will provide very good performance. You write: "the sampling frequency has to be several times the target frequency" but that is not true at all. It is possible to do under-sampling, listen on e.g. 144 MHz with an ADC running at 10 MHz. If you sample at 20 MHz and have an analog bandwidth of 200 ...


3

Velocity factor is a property of electromagnetic wave propagation, not wire. A transmission line (like coax) is a conduit for an electromagnetic wave in itself, so the velocity factor can be defined. In fact, the velocity factor can be derived from the lumped model of a transmission line: simulate this circuit – Schematic created using CircuitLab ...


3

The idea that a closed circuit — a loop — must be present for current to flow is a simplified description, which is only true for simple DC circuits. It's a simplification that works because in the DC case, if there is no complete circuit and yet current is flowing, then charge is accumulating somewhere, and the electric field from that excess of charge will ...


3

Filters. A filter is a circuit which is designed to react differently depending on the frequency. Every real (not mathematically ideal) circuit element does this to some degree, but filters are designed to have a very specific frequency response. Here's a very loose and incomplete high-level schematic of a simple radio receiver: simulate this circuit &...


Only top voted, non community-wiki answers of a minimum length are eligible